1. Field of the Invention
The present invention is directed to a method, in the form of a pulse sequence for operating a magnetic resonance imaging (nuclear magnetic resonance tomography) apparatus.
2. Description of the Prior Art
A pulse sequence is disclosed, for example, in U.S. Pat. No. 27,769,603 and is usually referred to by the acronym "FISP". For explaining a problem associated with this known method, an exemplary embodiment of the method disclosed in this patent is shown in FIGS. 1 through 4 herein. Each sub-sequence of this known method begins in a time segment I with a radiofrequency excitation pulse RF.sub.1 that has a flip angle of 90.degree.. The radiofrequency pulse RF.sub.1 is frequency-selective and is emitted in the presence of a slice-selection gradient GS2, so that only a selected slice of the examination subject is excited. In a time segment II, a dephasing of the nuclear magnetization in the read-out direction ensues by means of a gradient G.sub.R2. In the time segment II, further, a phase-coding gradient pulse G.sub.P2 as well as a gradient pulse G.sub.S3 directed oppositely to the slice-selection gradient pulse G.sub.S2 are activated. As a result of the gradient pulse G.sub.S3, the dephasing caused by the slice-selection gradient pulse G.sub.S2 is compensated.
In a time segment III, a read-out gradient pulse G.sub.R3 is activated and thus a rephasing of the nuclear spins is achieved, so that a nuclear magnetic resonance signal S1 arises. This nuclear magnetic resonance signal S1 is sampled and is employed in a conventional manner for producing an image.
In a time segment IV, a slice-selection gradient pulse G.sub.S4 in the positive slice-selection direction, a gradient pulse G.sub.P3 opposite the gradient pulse G.sub.P2 and a gradient pulse G.sub.R4 in the negative read-out direction are activated.
In a time segment V, a radiofrequency pulse RF.sub.2 having a flip angle of -90.degree. is activated in the presence of a slice-selection gradient pulse G.sub.S5 in the negative slice-selection direction. A new read-out interval is thus initiated. The illustrated pulse sequence is repeated n times with different values of the phase-coding gradient pulses G.sub.P2. The phase relation of the radiofrequency excitation pulse is thus inverted from pulse sequence-to-pulse sequence, so that the operational signs of the flip angles effected by the excitation pulse RF alternate. The spacing between two successive radiofrequency excitation pulses RF is referenced T.sub.R (repetition time). All gradients are switched such that their time integral over a repetition time T.sub.R yields zero.
A fast imaging is possible with this method since the repetition time T.sub.R can be made significantly shorter than the relaxation times T1 and T2.
In the illustrated pulse sequence, the magnetization is brought to 90.degree. by the first radiofrequency excitation pulse RF.sub.1 and to about 0.degree. by the second radiofrequency excitation pulse RF.sub.2, so that only every other radiofrequency excitation supplies a transient signal. A steady-state condition, wherein the flip angle moves between plus and minus 45.degree., is achieved only after a transient that lies roughly on the order of magnitude of T1 or T2. Until this steady state has been achieved, the nuclear magnetic resonance signal oscillates significantly and cannot be interpreted in practice.
Apart from the lost time for the signal acquisition that is caused by this transient, it also causes another disadvantage. In many instances, a preparation of the nuclear spins is desired, for example for influencing the contrast behavior or for suppressing specific spectral ranges. The effect of such a preparation, however, decays with the longitudinal or transverse relaxation time T1 or T2. Since a steady-state condition has not yet been established within this time given the illustrated FISP sequence, it is precisely the signals affected by the preparation that cannot be utilized.